CN1973375B - Method of separating layers of material using laser beam - Google Patents

Abstract

The material layer is separated from the substrate using a lift-off method by irradiating the interface between the material layer and the substrate. According to one example process, the layer is divided into a plurality of segments corresponding to the dies on the substrate, and the uniform beam spot is shaped to cover an integer number of segments.

Description

A method of separating layers of material using a laser beam

This application claims the benefit of co-pending US Provisional Patent Application No. 60/557,450, filed March 29, 2004, which is hereby incorporated by reference in its entirety.

technical field

The present invention relates to the separation of layers of material, and more particularly to the separation of layers of material, such as substrates and films grown on substrates, by irradiating interfaces between layers.

Background technique

GaN/InGaN-based light emitting diodes (LEDs), known as "blue LEDs", hold great promise. The practical applications of these GaN/InGaN-based LEDs have been expanded to include products such as mobile phone key-pads, LCD backlights, traffic lights, commercial signs, automotive lights, outdoor full-color display panels, home lighting fixtures, and the like. In these and other applications, these high-brightness LEDs can replace traditional light sources such as incandescent and fluorescent lamps. Blue LEDs are characterized by high light output (energy saving, high efficiency) and longer operating lifetime at lower energy input than conventional light sources. Their high performance and reliability make them promising successful replacements for conventional light sources; however, improvements to current LED designs are needed to overcome currently known limitations and inherent shortcomings. Better and more sophisticated manufacturing techniques can help facilitate the design of blue LEDs by cutting waste, increasing yields and allowing more advanced and complex or improved designs, thereby promoting more technical flexibility in Design for Manufacturing (DFM) . This improved manufacturing technique simplifies and reduces their manufacturing costs.

Blue LED’s can be fabricated by depositing GaN/InGaN layers on a sapphire substrate. Once the LEDs have been fabricated, the wafer is separated into a number of individual dies. A current die separation process involves the following steps. First, the wafer is thinned to a thickness of less than 100 μm by grinding and polishing the backside of the wafer. Next, the wafer is mounted on a dicing tape and then scribed along the inter-die streets by a diamond scribe tip or a UV laser beam.

Finally, the wafer is fractured along the scribe line by a fracturing tool. After fracturing, the dicing tape is stretched to physically separate the die from each other, allowing subsequent automated pick or place operations. This process is referred to as "scribing and cutting" die separation.

The major cost of LED manufacturing is the sapphire thinning and dicing-cutting operations. A method called LED lift off greatly reduces the time and cost of the LED manufacturing process. LED lift-off eliminates dicing by enabling manufacturers to grow, for example, GaN LED film devices on sapphire wafers, and then transfer the thin film devices into heat sinks for electrical interconnects. In this process, a laser beam is shone across the backside of the sapphire wafer, debonding and transferring the GaN LED devices to a substrate that is then packaged into a heat sink and/or reflector. Using special wafers, the sapphire growth substrate can be reused and LED manufacturing costs can be reduced. Additionally, this approach is fast, increases the light output of the LED, and has low operating costs due to low stress on the UV laser.

Current GaN LEDs designs have inherent limitations that hinder efforts to improve performance and reliability. The design also has problems with electrostatic discharge. As shown in FIGS. 1A and 1B , a blue LED 10 may include multiple layers of InGaN and GaN based layers 12 a , 12 b , 12 c grown heteroepitaxially on a silicon carbide or sapphire wafer substrate 14 . Since the sapphire wafer is a natural insulator, the current is supplied through the horizontal electrode structure. Due to the high resistance of the p-GaN layer 12a, a Ni/Au thin film 16 is deposited over the p-GaN to facilitate current spreading. However, the horizontal structure has some disadvantages.

)。大约25％LED自身发出的光被Ni/Au膜16吸收。此外，显著百分比的发射光在通过传输通过蓝宝石时损失。由于蓝宝石晶片与其周围之间折射率的差异，导向蓝宝石衬底14的一些光被反射回正面。Ni/Au薄膜16也吸收了大部分这种反射的输出光。First, the Ni/Au film 16 absorbs a substantial portion of the LED light output. Because Ni/Au film 16 has limited transmittance to emitted light, in order to make it transparent for LED light, Ni/Au film 16 is very thin (usually less than 100

). About 25% of the light emitted by the LED itself is absorbed by the Ni/Au film 16 . Furthermore, a significant percentage of emitted light is lost in transit through sapphire. Due to the difference in refractive index between the sapphire wafer and its surroundings, some of the light directed toward the sapphire substrate 14 is reflected back to the front side. The Ni/Au film 16 also absorbs most of this reflected output light.

Second, the Ni/Au film 16 is sensitive to moisture, resulting in performance degradation over time. To maintain the transparency of the film, thin Ni/Au is deposited by metal evaporation followed by heat treatment in ambient air or _O2 environment. The Ni/Au film 16 forms an oxidized compound, _NiOx having an Au-rich structure. When moisture permeates through the oxide film during long-term operation, the LED device 10 will be damaged.

Third, the Ni/Au film 16 experiences a decrease in process efficiency of the InGaN MQW light emitting layer 12b due to the current crowding effect. Since the Ni/Au film 16 through which the current propagates has lower resistance than the n-GaN layer 12c, current may be congested in the region 18 near the n(-) electrode 20 (see FIG. 1A). Therefore, the phenomenon of current congestion may prevent uniform use of the active InGaN region, resulting in low light output efficiency and low reliability due to uneven use of the active region.

Fourth, the horizontal electrode structure can generate a current bottle neck effect, resulting in low reliability. Current supplied through p(+) electrode 22 propagates across Ni/Au film 16, and flows from p-GaN 12a through InGaN 12b to n-GaN 12c. Since the n(-) electrode 20 is horizontally located at the n-GaN 12c, the current is bottlenecked in the region 24 of the electrode 20 (FIGS. 1A and 1B).

LEDs with vertical electrode structures overcome many of the disadvantages of horizontal LED structures. As shown in FIG. 2, an LED 30 having a vertical structure includes GaN layers 32a, 32b, 32c from a sapphire substrate to a conductive substrate 34, eg a silicon wafer. The vertical electrode structure can cancel the Ni/Au film, greatly increasing the light output. The vertical configuration allows the deposition of a metallic reflective layer 36, minimizing light loss when passing through the sapphire in the horizontal configuration. Vertical architecture also improves reliability and performance by reducing or eliminating current congestion and bottlenecks. A factor in constructing the vertical LED structure is the successful lift-off process of the GaN layer from the epitaxial sapphire wafer to the conductive silicon wafer.

An example of a high-brightness vertical LED configuration is shown in Figure 3. First, GaN layers 32 a , 32 c are deposited on a sapphire wafer 38 . After the metal thin film reflector 36 is deposited on the p-GaN, a Si substrate, or any other conductive substrate 34 (including GaAs substrate and thick metal film) is then bonded over the metal thin film reflector. The sapphire wafer was removed by UV laser lift-off as described below. An n(-) electrode is deposited on the n-GaN layer and a p(+) electrode is deposited on the Si wafer. Since the n-GaN layer has lower resistance than the p-GaN layer, a thin Ni/Au film is no longer required. Thus, the current is more evenly spread without congestion or bottleneck effects. Under the vertical structure, the elimination of the troublesome Ni/Au film leads to increased performance and reliability of LEDs.

Vertical structures can be produced using a UV laser lift-off process. One approach to UV laser lift-off involves selectively irradiating the GaN/sapphire interface with UV laser pulses, using the difference in UV light absorption between the GaN (highly absorbing) thin-film layer and the sapphire substrate. Typically, a GaN layer is grown heteroepitaxially on a sapphire wafer. To facilitate GaN crystal growth, the buffer layer may be deposited at a lower temperature of about 300°C. Although the buffer layer is helpful when growing the GaN layer at high temperature, the buffer layer contains a very high density of various defects due to the large lattice mismatch. Crystal defects such as dislocations, nanopipes and inversion domains raise the surface energy and thus increase the absorption of incident UV light. The incident laser beam used for the lift-off process has an energy density well below the absorption threshold of the sapphire wafer, allowing it to propagate through without causing any damage. In contrast, the laser fluence is high enough to cause photo-induced disintegration at the interface, which allows debonding of the interface.

There have been studies on the UV laser lift-off process. Kelly et al. showed that GaN was decomposed by laser irradiation through transparent sapphire using a 355 nm Q-switched Nd:YAG laser. (See M.K. Kelly, O. Ambacher, B. Dalheimer, G. Groos, R. Dimitrov, H. Angerer and M. Stutzniann, Applied Physics Letter, Vol. 69, p. 1749, 1996). Wong et al. used a 248 nm excimer laser to achieve separation of ~5 μm thin GaN films from sapphire wafers. (See W.S. Wong, T. Sands and N.W. Cheung, Applied Physics Letter, Vol. 72, p. 599, 1997). Wong et al further developed the lift-off process on GaN LEDs using an excimer laser at 248 nm (W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano and N. M. Johnson, Applied Physics Letters, Vol. 75, p. 1360, 1999). Kelly et al. also demonstrated the use of raster scanning of a Q-switched 355 nm Nd:YAG laser to lift off a 275 μm thick free-standing GaN film. (M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gorgens, O. Ambacher and M.Stutzmann, Japanese Journal of Applied Physics, Vol. 38, p. L217, 1999). Kelly et al. also reported that they had difficulty overcoming extended cracks in thick GaN films during the laser lift-off process due to high residual stress from the GaN-sapphire wafer. As above, in this study the authors had to heat the GaN/sapphire wafer to 600°C, but they could not completely counteract the problem of cracking due to residual stress.

)和蓝宝石晶片()之间热膨胀系数的巨大差异(参见表1)，当GaN/蓝宝石晶片从MOCVD工艺的高温冷却至环境温度时存在高水平的残余应力，如图4B所示。残余应力包括GaN上的压缩残余应力40和蓝宝石上的拉伸残余应力42。Despite the advantages from UV laser lift-off, poor productivity due to low process yield has limited the fabrication of GaN LEDs. The low yield is due in part to the high residual stress in the GaN-sapphire wafers resulting from the metal-organic chemical vapor deposition (MOCVD) process. The MOCVD process requires an activation temperature above 600°C. As shown in FIG. 4A , GaN and InGaN layers 32 are deposited on a sapphire wafer 38 by MOCVD process. Because in GaN(

) and sapphire wafers ( ) (see Table 1), there is a high level of residual stress when the GaN/sapphire wafer is cooled from the high temperature of the MOCVD process to ambient temperature, as shown in Figure 4B. The residual stresses include compressive residual stress 40 on GaN and tensile residual stress 42 on sapphire.

Table 1 Different material properties of GaN and sapphire

When an incident laser pulse with sufficient energy hits the GaN/sapphire interface, the irradiation causes the interface to stick instantaneously. Because the incident laser pulse has a finite size (typically much smaller than 1 cm ² ), it produces only a small portion of the debonded or peeled interface. Because the periphery of the debonded area still has a high level of residual stress, it creates stress concentrations at the bonded/debonded boundary, leading to cracks at the boundary. Such residual stress-related cracks have been one of the obstacles for the UV laser lift-off process.

Currently, there are different approaches for performing laser lift-off processes on GaN/sapphire wafers. One method involves raster scanning of a Q-switched 355 nm Nd:YAG laser (see for example M.K. Kelly, R.P. Vaudo, V.M. Phanse, L. Gordons, O. Ambacher and M. Stutzmann, Japanese Journal of Applied Physics, Vol. 38 , p. L2l7, 1999). This lift-off process using a solid-state laser is illustrated in Figure 5A. Another approach uses a 248 nm excimer laser (see for example W.S. Wong, T. Sands, N.W. Cheung, M. Kneissl, D.P. Bour, P. Mei, L.T. Romano and N.M. Johnson, Applied Physics Letters, Vol. 75, p. 1360, 1999). This lift-off process using an excimer laser is illustrated in Figure 5B.

Both processes use raster scanning, which involves translation of a laser beam 44 or a GaN/sapphire wafer target 46 as shown in FIG. 6 . A problem associated with the raster scan method is that it requires overlapping exposures to cover the desired area, resulting in multiple exposures48 for specific locations. In the above two methods, laser lift-off of GaN/sapphire is a single-pulse process. Unnecessary multiple exposures in localized areas increase the likelihood of cracks by inducing excessive stress on the film.

As shown in Figure 7, raster scanning also involves scanning the laser beam 44 from one end to the other, gradually separating the GaN/sapphire interface from one side to the other. This side-to-side release of residual stress induces a large difference in stress levels at the interface 50 between the separated and non-separated regions, ie the interface between scanned and unscanned regions. The difference in residual stress levels at interface 50 increases the likelihood of Model I and Model II crack propagation. Although the illustrations of Figures 6 and 7 are based on a process using a solid state laser, raster scanning with an excimer laser will produce similar results.

Currently, a typical size for a sapphire wafer is two inches in diameter, but other sizes (eg, three and four inch wafers) may be used for heteroepitaxial growth of GaN. For GaN/sapphire wafers, the level of residual stress varies from wafer to wafer, and compressive and tensile residual stress can exist together. The presence of residual stress can be observed by wafer warping or bowing. As mentioned above, when the laser lift-off process relaxes a large portion of the continuous GaN/sapphire interface, severe strain gradients can be generated at the boundary between the debonded and bonded interfaces. This strain gradient may cause crack expansion in the GaN layer.

When the target is irradiated with strong laser pulses, the shallow layer of the target can be instantly evaporated into high-temperature and high-pressure surface plasma. This phenomenon is called ablation. The plasma generated by the ablation then spreads to the surroundings. The extension of the surface plasmon may induce a shock wave that transfers the pulse to the target. When passing the laser light through a transparent material placed over the target, the ablation can be confined between the two materials. During this confined ablation, the trapped plasma at the interface may generate a larger shock wave, increasing the shock pressure. The explosive shock wave from the confined ablation at the GaN/sapphire interface will not only cause the detachment of the GaN layer from the sapphire substrate, but may also fracture the GaN layer near the laser beam spot (see e.g. P. Peyre et al., Journal of Laser Applications, pp. 8, pp. 135-141, 1996).

Therefore, there is a need for an improved method of detaching GaN thin films from sapphire wafers to solve the problems associated with residual stress leading to low yields due to fracture of the detached film layers. There is also a need for a process that can be extended to any lift-off application to address one or more of the above-mentioned problems.

Contents of the invention

According to one aspect of the present invention, there is provided a method of separating at least one layer of material from a substrate, the method comprising: providing first and second substrates and at least one layer of material between the substrates, said at least one layer of material is divided into a plurality of segments separated by spaces; forming a beam spot using a laser, wherein said beam spot is shaped to cover a region comprising an integer number of said segments and said region within said region any number of said intervals between segments; and using said beam spot to irradiate an interface between said first substrate and said segments, wherein a plurality of regions including said integer number of said segments is performed said irradiating until said first substrate is separated from all said sections.

According to another aspect of the present invention, there is provided a method of separating at least one layer of material from a substrate, the method comprising: providing a substrate on which at least one layer of material is formed; forming a uniform beam spot using at least a laser and a beam homogenizer and using a single pulse of said uniform beam spot to irradiate the interface between said layer and said substrate with a substantially uniformly distributed laser fluence, wherein said laser fluence of said uniform beam spot is sufficient at said The interface induces a detonation shock wave, wherein the detonation shock wave separates the first substrate from the material layer.

According to yet another aspect of the present invention, there is provided a method of separating at least one layer of material from a substrate, the method comprising: providing a substrate on which at least one layer of material is formed; attaching a second liner to said at least one layer of material; and irradiating the interface between the first substrate and the material layer by exposing the interface to a laser at a range of angles relative to the interface, thereby separating the first substrate from the material layer substrate.

Description of drawings

These and other features and advantages will be better understood by reading the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a schematic diagram illustrating a cross-section of a conventional GaN LED with a horizontal electrode structure.

Figure 1B is a top view of the GaN LED shown in Figure 1A.

Figure 2 is a schematic diagram illustrating a cross section of a GaN LED with a vertical electrode structure.

FIG. 3 is a flowchart illustrating the construction of a GaN LED with a vertical electrode structure.

FIG. 4A is a schematic diagram illustrating a GaN/sapphire wafer during an MOCVD process.

4B is a schematic diagram illustrating the formation of residual stress on a GaN/sapphire wafer after the MOCVD process.

5A is a schematic diagram illustrating a conventional method of laser lift-off on a GaN/sapphire wafer using a Q-switched 355 nm Nd:YAG laser.

5B is a schematic diagram illustrating a conventional method of laser lift-off on a GaN/sapphire wafer using a 248 nm excimer laser.

Figure 6 is a schematic diagram illustrating the raster scanning of a Q-switched 355nm Nd:YAG laser on a GaN/sapphire LED wafer and the resulting multiple exposures.

7 is a schematic diagram illustrating raster scanning on a GaN/sapphire LED wafer and the resulting stresses that create a high probability of Model I and Model II cracks at the interface.

Figure 8 is a schematic illustration of the use of laser pulses to induce shock waves to separate layers, consistent with one embodiment of the present invention.

Figure 9 is a schematic diagram illustrating a cross-section of laser exposed regions and layer separation, consistent with an embodiment of the present invention.

10A-10C are schematic diagrams showing the effect of different laser fluences.

11 is a schematic diagram of a wafer, consistent with one embodiment of the present invention, illustrating the selective ablation of the GaN layer over spaces to divide the GaN layer into multiple dies, leaving a complete sapphire wafer.

12 is a schematic diagram of a beam delivery system, consistent with another embodiment of the invention, illustrating the projection of a uniform beam and a representative beam profile shown along the beam path.

13 is a schematic diagram of a wafer illustrating laser lift-off exposure using a step-and-repeat process, consistent with another embodiment of the present invention.

Figure 14 is a schematic of a wafer illustrating a single pulse exposure on a 3x3 LED array using a step-and-repeat lift-off process.

15 is a schematic diagram illustrating a laser lift-off process that combines separation of residual stress and precise step-and-repeat laser beam exposure consistent with yet another embodiment of the present invention.

Figure 16 is a photograph of a wafer with GaN selectively removed by a solid state UV laser with a variable astigmatism focal spot.

Figure 17 is a schematic diagram illustrating concentric or helical laser ablation exposure with a square beam spot, consistent with yet another embodiment of the present invention.

Figure 18 is a schematic diagram illustrating concentric or helical laser ablation exposure with a circular beam spot, consistent with yet another embodiment of the present invention.

Figure 19 is a schematic diagram illustrating concentric laser ablation exposure with a variable annular beam spot, consistent with yet another embodiment of the present invention.

Figure 20 is a diagram illustrating a laser lift-off process, consistent with yet another embodiment of the present invention.

Detailed ways

DETAILED DESCRIPTION OF THE INVENTION Exemplary embodiments of processes consistent with the present invention are described that solve problems associated with existing lift-off processes and increase productivity. The application of the present invention is not limited to the following exemplary embodiments. Although the exemplary embodiments refer to GaN and sapphire and a GaN/sapphire interface, other types of substrates and layers of materials known to those skilled in the art may be used. Additionally, a sacrificial layer may be provided between GaN (or other material layer) and sapphire (or other type of substrate).

Referring to FIG. 8 , a laser may be directed through at least one layer of substrate material 102 to at least one target material 104 , thereby separating the materials 102 , 104 . In an exemplary embodiment, substrate material 102 is sapphire and target material 104 is gallium nitride (GaN). Separation of the materials 102 , 104 is accomplished by using a laser fluence sufficient to induce a shock wave at the interface 106 of the target 104 and substrate material 102 , thereby instantaneously debonding the target 104 from the substrate material 102 . Shock waves may be generated by explosive expansion of the plasma 108 at the interface as a result of the increased density of the ionized vapor when the plasma temperature is rapidly raised. The laser fluence may be in a range sufficient to induce a force Fa on the target 104 to cause separation without fracturing. The applied force Fa can be expressed as follows:

P _P (GPa)＝C[I _r (GW/cm ² )] ^1/2

F _a (N) = P _P (GPa) A _r (cm ² )

where P _P is the peak pressure induced by the explosive shock wave, C is the efficiency and geometry factor, I _r is the irradiance of the incident laser beam, F _a is the applied force, and _Ar is the irradiated area.

As the plasma 108 expands, as shown in FIG. 9, the laser-exposed area acts as a curved arm that turns at the edge of the laser-exposed area. For example, the force required to rupture or fracture (F _r ) can be considered as a two-point bend test and can be expressed as follows:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; \&Proportional;</span>\frac{{\mathrm{<span\; class="notranslate">\; wd</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; \&sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}$

where d is the thickness of the target 104, w is the width of the applied force or the width of the laser pulse, L is the length of the applied arm or the half-width of the laser pulse, and _σr is the modulus of GaN rupture or fracturing stress. To increase the force (F _r ), the width w of the laser pulse can be increased and the half-length L of the laser pulse can be decreased, forming a linear beam. A linear beam can be scanned across the target to minimize bending moments during irradiation.

For example, at laser fluences below the GaN ablation threshold (~0.3 J/ ^cm2 at 248 nm), instantaneous separation of the GaN/sapphire interface 106, as shown in Figure 10A, cannot be successfully achieved. Although GaN may decompose at the ablation threshold, this alone cannot achieve instantaneous separation of the interface 106 because there is no driving force without ablation, ie, a shock wave from the expanding plasma. Conversely, applying too high a laser fluence may generate excessive explosive stress wave propagation, causing cracks and fractures on the target 104 (eg, GaN film), as shown in FIG. 10C . When the irradiated laser fluence is optimized as shown in FIG. 10B , the force generated by the shock wave is sufficient to separate the layers 102 , 104 at the interface 106 , but insufficient to induce fracture at the target 104 . According to an exemplary embodiment using GaN and sapphire, the optimum range of laser fluence is between about 0.60 J/cm ² -1.5 J/cm ² .

The parameters of laser irradiation, such as wavelength and energy density, depend on the type of material to be separated. For example, the optimal laser fluence for separating GaN from sapphire is discussed above. A laser wavelength of 248nm is also required for separating GaN from sapphire. It is well known to those skilled in the art that the photon energy at 248nm (5eV) is between the energy gaps of GaN (3.4eV) and sapphire (9.9eV). This suggests that 248nm radiation is absorbed better in GaN than in sapphire, and that the selective absorption causes ablation leading to separation.

Those skilled in the art recognize that other laser wavelengths can be used to separate other types of materials. For example, a buffer layer can be used between the sapphire substrate and the GaN layer to facilitate the epitaxial growth of GaN. Examples of the buffer layer include a GaN buffer layer and an aluminum nitride (AlN) buffer layer. In the case of using the AlN buffer layer, since the photon energy (6.4eV) of the 193nm laser is between the energy gaps of sapphire (9.9eV) and AlN (6.1eV), a 193nm laser can be used.

According to one embodiment of the present invention, as shown in FIG. 11 , prior to lift-off or separation from a substrate 110 such as a sapphire wafer, the one or more layers to be separated (e.g., GaN films or layers) may be formed into smaller Area or section 112 . In one embodiment, for example, segments 112 may be separated, corresponding to LED dies. Formation of the segment 112 reduces fracture induced at the interface due to residual stress and shock waves during the lift-off process. The segment 112 of the GaN film is less affected by the induced residual stress from its surroundings. Furthermore, segments 112 have insignificant amounts of residual stress and strain that the thin GaN film in these segments 112 can withstand.

According to one embodiment, GaN/sapphire LED wafer 11 contains a symmetrical and repeating pattern of segments 112 to form small-sized LED dies, typically a few hundred square microns or rectangular in size. The symmetrical and repeating segments 112 may be separated, for example, by spaces 114, which define the boundaries of the LED dies and provide sacrificial space for die separation, for example using a dicing and cutting process. Although segment 112 in the example embodiment corresponds to a single square die, those skilled in the art will recognize that other structures and shapes may be formed, such as a rectangular shape.

The GaN film may be separated into segments 112 by selectively removing or etching the GaN layer over spacers 114 . One method of selectively removing the GaN layer over spacers 114 is by reactive ion etching, which is generally known to those skilled in the art. This approach has several disadvantages, including slow etch rates and the necessary handling of hazardous chemicals. Another approach involves selective ablation by a solid-state UV laser using a variable astigmatism focal spot formed by an anamorphic beam delivery system, as disclosed in U.S. Patent Application Serial No. 10/782,741, which is incorporated herein by reference in its entirety . The variable astigmatism focus spot can effectively adjust its size to an optimal laser energy density, so as to selectively ablate the GaN layer on the spacer 114 without affecting the sapphire substrate (see FIG. 11 ). This selective GaN ablation exploits the large difference in ablation threshold between GaN (0.3 J/cm ² at 248nm) and sapphire (over 2 J/cm ² at 248nm).

According to another approach, patterned laser projection (for example using an excimer laser) can be used for etching. Patterned excimer laser beams can also be used to dry pattern GaN spacers or devices, or pattern such as ITO, metallization, or dielectric insulating layers, or other thin films for other devices or conductive or insulating layers . As an alternative to removing portions of the continuous GaN film to form segments 112 and spaces 114 , GaN may be formed (eg, grown) on substrate 110 as segments 112 and spaces 114 . However, it may be more economical to grow a continuous GaN film than to grow a GaN layer with a pattern of spaces 114 and segments 112 .

According to another approach, after the substrate 110 has been removed, the spaces 114 between the segments 112 may be widened, for example using reactive ion etching. Re-etching of spacers 114 may reduce or eliminate the possibility of leakage current at the sidewalls of region 112, such as the n-GaN and p-GaN junctions.

Segment 112 (eg GaN layer) may be separated from substrate 110 (eg sapphire wafer) by irradiating the interface between substrate 110 and segment 112 using a lift-off process. An exemplary laser lift-off process may use a single-pulse process with a uniform beam spot and an energy density sufficient to induce a shock wave as described above. The single-pulse process avoids overlapping exposures at the interface between the substrate 110 and the segment 112 and thus minimizes fracturing. The interface between the layers to be separated can be irradiated with a uniform beam spot to substantially eliminate density gradients, thereby facilitating effective exfoliation. Beam homogenizers can be used with UV solid-state lasers and excimer lasers to create a uniform beam spot for the lift-off process. An exemplary embodiment uses a 248nm KrF excimer laser. The gaseous laser medium with electric discharge produces high average power at a large original beam diameter. The application of the beam homogenizer is effective under the large and strong original beam of the excimer laser. In addition, providing a uniform distribution of laser fluence in the beam spot advantageously produces efficient ablation in the area under single pulse irradiation.

Figure 12 illustrates an example of the projection of a uniform beam by near-field imaging and shows a representative beam profile along the beam path. The raw beam 120 from the excimer laser 120 has a Gaussian distribution on the short side/a flat top distribution on the long side. The beam homogenizer 122 (such as a multi-array structure) makes the profile of the gradient raw beam into a square flat-top profile. The homogenized beam is clipped by a mask 124 (eg, a rectangular variable aperture) to use an optimal portion of the beam, eg, projected onto the LED target wafer 116 by near-field imaging using a beam imaging lens 126 . Therefore, the edge resolution of the uniform beam spot 130 on the LED die 116 is improved. Although one configuration of the beam delivery system is shown in this exemplary embodiment, those skilled in the art will recognize that other configurations can be used to generate and deliver a uniform beam. Although the exemplary embodiment shows a mask 124 having a rectangular aperture, any shape mask can be used for near-field imaging.

According to one exemplary method, lift-off exposure is performed using a step-and-repeat process. As shown in FIG. 13 , the homogenized beam spot 130 is shaped to include an integer number of segments 112 (eg, corresponding to an integer number of LED dies). Based on the size of the discrete segments 112, the beam spot 130 is precisely sized to include multiple discrete segments 112, such as a 3x3 array. A single pulse exposure can be used to irradiate the interface between the substrate and an integer number of segments 112, and the process can be repeated for each set of segments (ie, dies). The ordinal numbers in FIG. 13 indicate an exemplary sequence of step-and-repeat processes. Stitching of beam spots 130 may be performed as irradiation is repeated for each set of segments 112 . Advantageously, stitching can be done within the space 114 to avoid possible damage in the active LED area. In an exemplary process, the stitching of the beam spot 130 is maintained within about 5 μm.

Figure 14 illustrates an example of single-pulse exposure of an LED lift-off wafer by a 248nm excimer laser. In Figure 14, the homogenized beam spot covers nine (9) LED dies and the disbonded GaN/sapphire interface appears brighter.

Exemplary laser lift-off exposure does not require heating of the LED wafer to counteract residual stress due to precisely controlled exposure in a small area by a single pulse. Exposure can be performed at room temperature. Because the lift-off exposure laser passes through the sapphire wafer, damage or debris on the sapphire surface causes shielding at the GaN/sapphire interface, causing defects at the lift-off interface. The surface of the sapphire can be polished to remove any chips or particles. Lift-off exposure can also be applied to the target at a range of different angles, which reduces or eliminates shadowing effects.

The exemplary process described above can improve the productivity and yield of the UV laser lift-off process for successful industrial applications. An exemplary method consistent with the present invention combines separation of residual stress on the LED wafer and uniform beam laser exposure. Selective etching of the GaN layer on the spacers divides the film into small regions on which residual stress from the surroundings has minimal influence. In addition, the small area itself has minimal residual stress, which will hardly affect the GaN film during lift-off exposure. A homogenized beam distributes substantially uniform laser fluence in the beam spot. Precise laser exposure with a homogenized laser beam enables correct peeling with optimal laser fluence.

Figure 15 illustrates an exemplary lift-off process. After growing one or more GaN layers 132 on the sapphire substrate 110, a protective coating 135 may be applied to prevent deposition of laser-generated debris on the GaN layer 132 after laser scribing. GaN layer 132 is selectively removed by laser scribing or reactive ion etching to form spacers 114 and segments 112 . After removal of the protective coating 135 , a conductive substrate 134 is bonded on the GaN layer 132 . Conductive substrate 134 may be any type of conductive ceramic and metal including, but not limited to, Si, Ge, GaAs, GaP, copper, copper tungsten, and molybdenum. A reflective layer (not shown) may also be formed between GaN layer 132 and conductive substrate 134 . Then, the sapphire substrate is removed by a laser lift-off process. After laser lift-off, the GaN surface can be processed for deposition of electrode metal films or other required steps. Finally, the wafer is separated, eg, between segments 112 to form individual LED dies.

An example of selective removal of a GaN layer on an actual wafer is shown in Figure 16, where GaN is removed using a solid-state UV laser using a variable astigmatism focal beam spot to provide high-speed laser dicing. In this embodiment, a monolithic GaN layer without die or spacer patterns is first grown on a sapphire wafer. The LED die size is defined by the lines cut by the laser. In this embodiment, the width of the selective removal or laser dicing is only about 5 [mu]m, which minimizes the loss of actual places on the wafer.

In conductive substrates for liftoff, molybdenum has desirable properties such as coefficient of thermal expansion matching (CTE), high reflectivity in the blue light spectrum and high strength with low ductility. Molybdenum has a CTE (4.8×10 ^-6 /K) relatively close to that of GaN (5.6×10 ^-6 /K). Metal compounds such as PdIn and SnAu can be used to bond conductive substrates on GaN. When using these bonding metals, for example, the GaN and substrate are heated to about 400°C. A large CTE mismatch between the exfoliated substrate of the GaN layer can introduce another high level of residual stress, which is detrimental to the bonding process. For example, although Cu has large thermal and electrical conductivity, it is not advisable as a lift-off substrate under a 2-inch GaN/sapphire wafer because of its high CTE (16.5×10 ⁻⁶ /K).

Molybdenum has a reflectivity of approximately 55% in the blue spectral region from 350nm-450nm. This value is comparable to other metals. For example, the reflectance values of most metals at 410nm are as follows: gold (37%), copper (51%), nickel (50%), platinum (57%), iron (58%), chromium (70%), silver ( 86%), Aluminum (92%). While the comparable reflectivity enables molybdenum to be used directly as a reflector (ie without a separate reflective layer), light output can be maximized by depositing metal films with high reflectivity, such as aluminum and silver. A highly reflective layer between GaN and Mo, for example, can increase the performance of blue LEDs without introducing high levels of residual stress. For example, aluminum can be deposited on the GaN surface by sputtering to form a reflective layer. Because oxidation of the aluminum film is detrimental to adhesion to the molybdenum substrate, another metal film can be deposited to prevent oxidation and enhance adhesion. Examples of metal films that do not oxidize and allow molybdenum to adhere to the aluminum film include, but are not limited to, tin, zinc, lead, and gold.

Molybdenum also provides advantages during the die separation process. Mainly due to the high ductility of the metal film, conventional diamond saws or diamond scribes are difficult to use to separate the metal film. Laser cutting and dicing is an optional method for die separation. However, since mechanical fracture is difficult on ductile substrates, highly ductile metal films such as copper require 100% severing to separate. Thus, laser severing causes handling problems because it does not maintain the integrity of the small die after dicing. Molybdenum has high strength and low ductility. These unique mechanical properties of molybdenum facilitate mechanical fracture, even when laser scribed up to approximately 90% of its thickness.

According to another exemplary approach, laser lift-off exposure can be combined with high-speed motion-controlled techniques to maximize throughput. When laser ablation uses step-and-repeat exposures with precisely designed beam stitching, it is desirable to trigger the laser exactly on the target. The fastest possible speed of the step-and-repeat process is also advantageous for increasing productivity. A special function of the motion control can be used to compare the position of the motion table and send a trigger signal to the laser at a predetermined position. The technology is called "position compare and trigger" or "fire on fly". As the motion table moves continuously, a processor in the motion controller constantly compares the coded counter to the user-programmed value and sends a trigger signal to the laser with a matching value. Therefore, instead of stopping the motion table for step-and-repeat processes, the table can be moved in continuous motion, ie the laser is activated while it is in motion. For example, a laser pulse repetition rate of 200 Hz with a uniform beam spot size of 1 × ¹ mm can perform a lift-off process of a 2-inch diameter LED wafer in about 1 minute when the lift-off process uses motion-on-start technology.

Although the exemplary embodiment involves forming the spaces 114 and segments 112 prior to performing the lift-off process, the techniques described herein can also be used to separate continuous layers without first segmenting the layers. Although efficient separation of successive layers is possible, microcracks may form where laser pulses overlap.

Other exemplary methods may use unique techniques to scan the laser beam for exposure, such as irradiating in a concentric pattern. These techniques can be used to perform lift-off of one or more discrete layers or one or more continuous layers on a substrate. The residual stress of GaN/sapphire LED wafers has a concentric distribution, where tension and compression coexist. Laser exposure can cause large differences in stress levels at the interface between detached and non-separated regions, ie scanned and unscanned regions, when across the center of the wafer. Depending on the method, the scanning beam can be exposed circularly, helically, or helically, thereby relaxing the residual stress with position at the same level of stress. This approach reduces the stress gradient at the interface between scanned and unscanned regions. Alternatively, as described above, a line beam of dimensions such as to minimize the bending moment upon irradiation can be scanned across the interface.

FIG. 17 shows a concentric lift-off exposure with a square beam spot 150 . FIG. 18 shows a concentric lift-off exposure with a circular beam spot 152 . In one approach, the laser beam is stationary and the wafer is concentrically translated (eg, in a circular or helical pattern) for exposure. According to another approach, the beam can be moved (eg, in a circular or helical pattern) over a stationary wafer.

One way to move the circular beam is to use a galvanometer scanner, which precisely controls two mirrors in motion by rotating a motor. Other beam spots known to those skilled in the art may also be used, such as triangular, hexagonal or other polygonal shapes. In the case of a polygonal laser pattern irradiating a polygonal die, the beam can be moved in a circular or helical motion to cover the die or group of dies and separate the film from the substrate in a controlled pattern to relieve stress in a controlled manner.

Another alternative approach uses a variable annular beam spot to achieve concentric scanning. As shown in FIG. 19, the diameter of the variable annular beam spot 154 is gradually reduced to scan concentrically from the outer edge to the center of the wafer. A variable annular beam spot can be achieved by incorporating two tapered optics into the beam delivery system (BDS), where the distance between the two optics determines the diameter of the beam spot. The use of this concentrically moving annular beam spot provides a stable relaxation of residual stress during laser lift-off exposure.

Figure 20 illustrates a laser lift-off process for separating plated substrates, consistent with another embodiment. The sapphire wafer or substrate 110 on which GaN segments 112 are formed may be plated with a metal or metal alloy to form metal substrate 160 . Nickel or copper, or their alloys can be used for electroplating. Metal substrate 160 is then cut at locations 162 between segments 112, for example using a UV laser. A support film 164 may be fixed on the metal substrate 160, and then the sapphire substrate 110 may be separated using a laser lift-off process as described above. Portions of metal substrate 160 may then be removed using, for example, a post-laser lift-off process for contact metallization to form die 166 . Then, die 166 is separated. By dicing metal substrate 160 prior to laser lift-off, the integrity of die 166 may be maintained due to adhesion to sapphire substrate 110 . Those skilled in the art recognize that other materials can also be used to practice this method.

In summary, according to a method consistent with an aspect of the present invention, first and second substrates are provided and have at least one layer of material therebetween, the layer of material being divided into a plurality of segments separated by spaces. A beam spot is formed using a laser and shaped to cover an integer number of segments. The interface between the first substrate and the segment is irradiated with the beam spot. Irradiation is repeated for each of an integer number of sections until the first substrate is separated from all sections.

According to another method, a substrate is provided on which at least one layer of material is formed and a uniform beam spot is formed using at least a laser and a beam homogenizer. Using a single pulse of a uniform beam spot, the interface between the layer and the substrate is irradiated with a substantially uniform distribution of laser energy density, thereby detaching the layer from the substrate.

According to yet another method, a substrate is provided on which at least one layer of material is formed and the beam spot is formed using a laser. A beam spot is used to irradiate the first substrate and the interface between the layers. The interface is typically irradiated in a concentric pattern, thereby separating the layers from the substrate.

According to yet another method, a first substrate is provided on which at least one layer of material is formed and the layer of material is etched on the first substrate, dividing the layer into a plurality of spaced apart sections. A second substrate is attached to the segment and a uniform beam spot is formed using a laser. Shape the uniform beam spot to cover an integer number of segments. The interface between the first substrate and the segment is irradiated with a uniform beam spot. Irradiation is repeated for each of an integer number of segments. The first substrate is separated from all sections.

According to still another method, a first substrate on which at least one layer of GaN is formed is provided and at least one film is formed on the GaN layer. The film may include a reflective film. A second substrate comprising molybdenum was attached to the film and the interface between the first substrate and the GaN layer was irradiated, thereby separating the first substrate from the GaN layer.

While the principles of the invention have been described herein, it will be clear to those skilled in the art that such illustration has been made by way of example only, and not to limit the scope of the invention. Embodiments in addition to the exemplary embodiments shown and described herein are contemplated to be within the scope of the invention. Modifications and substitutions by those of ordinary skill in the art are considered to be within the scope of the present invention, which is not limited except by the following claims.

Claims (18)

1. one kind is separated the method for layer of material at least from substrate, and said method comprises:

Be formed with the substrate of at least one layer material above providing, wherein said at least one layer material be divided into by spaced-apart a plurality of sections;

At least use laser and beam-averaging device to form homogeneous beam spot, wherein the shape of said bundle spot is arranged to cover the said interval arbitrarily between the section described in the regional and said zone that comprises an integer said section; And

Use the pulse of said homogeneous beam spot; With the interface between said at least one layer material of equally distributed laser energy density irradiation basically and said substrate; With from the said at least one layer material of said substrate separation; Wherein for each all carries out said irradiation in said integer the said section, thereby the stitching of said bundle spot only takes place in the interval between said section, the said laser energy density of wherein said homogeneous beam spot is more than the ablation threshold of said at least one layer material; Thereby said laser energy density is enough at said interface induced explosion wave; Wherein said explosion wave separates said substrate from said at least one layer material, and wherein carries out said irradiation for a plurality of zones that comprise a said integer said section, until said sections from all, separates said substrate.

2. the process of claim 1 wherein that said substrate is a sapphire wafer, and wherein said at least one layer material comprises one deck GaN at least.

3. the method for claim 2, wherein said at least one layer material also comprises the GaN resilient coating.

4. the method for claim 3 wherein uses the 248nm PRK to form said homogeneous beam spot.

5. the method for claim 2, wherein said at least one layer material also comprises the AlN resilient coating.

6. the method for claim 5 wherein uses the 193nm PRK to form said homogeneous beam spot.

7. the process of claim 1 wherein that said laser energy density is at about 0.60J/cm ²-1.6J/cm ²Scope in.

8. the method for claim 1 also comprises:

On workbench, move said substrate and said at least one layer material; And

The position of said workbench is compared with predetermined value, wherein form the said interface between each in said bundle spot and a said substrate of irradiation and the said integer said section based on the said laser of said location triggered.

9. the process of claim 1 wherein that the said at least one layer material that substrate is provided and is formed on the said substrate comprises:

The said substrate that is formed with said at least one layer material on it is provided;

The said at least one layer material of etching, thus on said substrate, said at least one layer material is divided into by spaced-apart said a plurality of sections, and said section is corresponding with tube core; And

Additional second substrate on said section.

10. the method for claim 9 also comprises:

Before additional said second substrate, above said section and said interval, form metal substrate;

Position between said section is cut said metal substrate at least; And

Behind the said substrate of said segments apart, remove the said metal substrate of at least a portion.

11. the process of claim 1 wherein that said substrate is a semiconductor wafer, and this method is included in also and separates said section after separating said substrate, to form semiconductor element.

12. the process of claim 1 wherein that said laser is PRK, and wherein the said interface of irradiation comprises each for a said integer said section, said interface is exposed under the pulse of said PRK.

13. the method for claim 9, wherein said substrate comprises sapphire wafer.

14. the method for claim 13, wherein said at least one layer material comprises GaN.

15. the method for claim 14 also is included on the said at least one layer material that comprises GaN and forms one deck reflectance coating at least.

16. the method for claim 15, wherein said second substrate comprises molybdenum or its alloy.

17. the method for claim 16, wherein said reflectance coating are the aluminium films.

18. the method for claim 17, wherein said at least one layer material also comprises metal film on said aluminium film, appends on the said metal film comprising second substrate of said molybdenum.

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